1. Field of the Invention
The present invention relates to a method of manufacturing a silicon-based thin-film photoelectric conversion device having excellent performance. In particular, the present invention relates to a silicon-based thin-film photoelectric conversion device dramatically improved in production cost and production efficiency.
2. Description of the Background Art
In recent years, development of and increased production volume of solar cells using thin films containing such a crystalline silicon as polycrystalline silicon or microcrystalline silicon for example have received worldwide attention. A significant feature of this type of solar cells is in that reduction in cost and improvements in performance are simultaneously achieved by depositing semiconductor films or metal electrode films on a large-area and low-cost substrate with such a film deposition apparatus as plasma CVD apparatus or sputtering apparatus and thereafter separating and connecting resultant solar cells fabricated on the same substrate by means of laser patterning for example. Key apparatuses used in the process of manufacturing the devices, CVD apparatus for example, are increasing in cost to add the production cost of the solar cells. The increased cost is thus one of factors that hinder large-scale and widespread use of the solar cells. Here, it is noted that the terms “polycrystalline” and “microcrystalline” herein refer to crystals that are partially in an amorphous state.
Conventional manufacturing apparatuses of solar cells employ the inline system having a plurality of film deposition chambers (also referred to simply as chambers) linearly coupled to each other or the multi-chamber system having a plurality of film deposition chambers arranged around a central intermediate chamber. Regarding the inline system, however, the substrate is transported linearly, which means that the whole apparatus has to be stopped even if only a part of the apparatus fails to need maintenance. For example, the manufacturing process includes multiple steps for depositing i-layers that require maintenance most as compared with other components. Therefore, a disadvantage of the inline system is that, even if one film deposition chamber for forming i-layers requires maintenance, the entire manufacturing line has to be stopped.
Regarding the multi-chamber system, the substrate on which films are to be deposited is transported via the intermediate chamber to an appropriate one of the deposition chambers and a movable partition is provided between each of the deposition chambers and the intermediate chamber for keeping airtightness therebetween. Then, even if one of the deposition chambers fails, other deposition chambers are available, which means that the whole stop of the production does not occur. The manufacturing apparatus of the multi-chamber system, however, has a plurality of lines along which the substrate is transported via the intermediate chamber, resulting in an inevitable increase in complexity of the mechanical structure of the intermediate chamber. For example, the mechanism for maintaining the airtightness between the intermediate chamber and each of the deposition chambers while transporting the substrate is complicated to increase the cost. A further problem is that the number of film deposition chambers arranged around the intermediate chamber is limited in terms of space.
In view of the above-described problems, Japanese Patent Laying-Open No. 2000-252495 proposes a method of manufacturing a silicon-based thin-film photoelectric conversion device characterized in that a p-type semiconductor layer, an i-type microcrystalline silicon-based photoelectric conversion layer and an n-type semiconductor layer are deposited successively in the same plasma CVD film deposition chamber and the p-type semiconductor layer is deposited under the condition that the pressure in the film deposition chamber is at least 667 Pa. According to Japanese Patent Laying-Open No. 2000-252495, the proposed method can be used to manufacture photoelectric conversion devices having excellent performance and quality with a simple apparatus at a low cost and a high efficiency. The manufacturing method includes the steps of repeatedly forming the p-type semiconductor layer, the i-type microcrystalline silicon-based photoelectric conversion layer and the n-type semiconductor layer (hereinafter simply referred to as “pin layers” in some cases) within the same deposition chamber for the purpose of improving productivity. A problem of this method is therefore that the initial stages of the steps of depositing the p-layer and the i-layer are inevitably affected by n-type dopants remaining on the cathode and in the film deposition chamber that are dopants used in the preceding step of depositing the n-layer.
For example, the n-type dopants influence the p-layer by weakening the function of p-type dopants, so that p-type space charge necessary for manufacturing solar cells cannot be ensured. Consequently, there arise such adverse effects on various parameters of solar cells as decreases in open circuit voltage and polarity factor. In addition, regarding influences of the n-type dopants on the i-layer, Japanese Patent Laying-Open No. 2000-243993 shows that diffusion of remaining n-type dopants into the i-layer increases the recombination level in the i-layer to weaken the internal electric field, resulting in a considerable decrease in shortwave sensitivity of solar cells.